This invention relates to the field of power transmission and distribution equipment, more particularly to the field of protective devices for power transmission and distribution equipment, and more particularly still to the field of localized protection of power transmission and distribution equipment, such as transformers and surge arresters.
Transformers placed on poles, or at remote distribution sites, are subject to the ravages of weather upon the distribution system components. If lightning hits a wire or the transformer, an electrical surge will pass through the distribution conductor feeding the transformer. To help prevent these disruptive weather related occurrences from destroying the distribution equipment, surge arresters and fuses are placed in the distribution network immediately adjacent the transformer, or other component, which is to be protected. The surge protector is normally disposed in an electrical parallel configuration with the transformer. During normal, steady state operations, the surge arrester has a very high resistance to ground, and therefore, nearly all of the current in the distribution conductor leading to the transformer passes through the transformer. However, when a surge in the form of a high current spike is detected by the arrestor, the resistance of the arrester drops precipitously to a level substantially lower than that of the transformer, and the surge arrester diverts the surge to ground. Once the surge has passed or dissipated, the arrester once again returns to the high resistance state to re-energize the transformer.
Fuses are used to further protect the transformers located on poles or at remote locations in the distribution network. Power lines coming into the transformer are typically rated at between one half and 100 amps, supporting voltages of between 2400 and 38000 volts thereon. The fuses are commonly placed in a parallel electrical relation to the surge arrestor. The fuse, and transformer, are mounted in series and these two components in series are then mounted in an electrically parallel configuration to the surge arrestor. The arrestor provides protection to the transformer by diverting surges, such as those caused by lightning, to ground, rather than through the fuse and transformer combination and into the rest of the distribution network. The fuse provides long-term overload protection to the circuit, such as what occurs when a short appears as a result of a failure in the transformer or a long-term over load situation is present in the secondary circuit. However, the fuse is not intended to carry the lightning surge to ground. The prior art fuses cannot withstand the full surge current created during a lightning strike, and thus the surge arrestor must be placed in parallel with the series combination of the transformer and fuse to protect both the transformer and the fuse.
To physically locate the surge arrestor and fuse-transformer combination in a parallel electrical configuration, the surge arrestor and fuse are both placed upon the pole, pad, or other mounting location, or otherwise remotely located from the transformer tank, and the ground lead is run from the transformer tank to the surge arrestor. This arrangement leads to less protection of the transformer windings than would be present if the surge arrester were mounted directly on or in the transformer tank. It is known that the longer the length of the lead between the transformer and surge arrestor, the greater the likelihood of damage occurring to the transformer windings during a current surge condition. However, the prior art fuses dictate that the surge arrestor be remotely mounted so that the surge current does not pass through the fuse while protecting the transformer.
The individual fuse associated with a transformer must be sized to protect the transformer, and not prematurely open in response to rated amperage or slight overload conditions. Each transformer will have a specific rated primary amperage and voltage which must pass therethrough to provide the proper voltage and amperage on the secondary, or low voltage, side thereof. Likewise, as the rated amperage and voltage of the transformer varies from application to application, the fuse which protects the transformer must be sized to match the performance rating of the transformer. Therefore, the fuse manufacturer typically must supply a line, or group of fuses with different opening amperages, for proper transformer protection for any given range of transformers. These requirements are well known in the art, and handbooks, design manuals and government and industry standards are promulgated which dictate to designers the power absorption, time to open or blow, and overcurrent characteristics of fuses for high energy applications.
Prior art fuses employ a variety of materials as fusing links, or wires. The link, or wire, serves as the fusing element in the fuse which severs or opens in response to an overload or surge condition. The fusing link must be capable of withstanding, or carrying, a low current overload for some period of time without opening, but must also be capable of rapidly opening within a period of time as short as one one-hundredth of a second, when a fault or short circuit appears across the fuse. The required opening time for any given overload condition is governed by government standards which are well-known to those skilled in the art.
During surge openings, when the current passing through the fuse is the equivalent of a short circuit, the fusing link locally vaporizes at a point thereon between the button and leader. The leader is attached to one end of the fusing wire, in a large crimp. The cross-section of the crimp physically blocks off approximately two-thirds of the internal cross-sectional area of the auxiliary tube. The leader is spring-loaded with a spring flipper, so that upon a fuse opening, the severed wires will be physically pulled apart by the spring flipper actuating the leader outward the bottom of the fuse. Additionally, the crimp is substantially larger than the fusing wire, and the gasses generated during a fuse opening generate pressure within the tube, which bears upon the upper area of the crimp to create a differential pressure thereon to help speed up the ejection of the severed fuse link from the auxiliary tube.
To obtain the above-referenced fuse opening characteristics, fuse designers must use materials with well-known properties and then physically size and shape the fusing link to accommodate the limitations of the fusing material while still obtaining the required fusing characteristics. One very common fusing link material is tin. Tin is a relatively inexpensive material which has well-known fusing properties. However, tin has several disadvantages when used as a fusing link material. When tin wire is required to sustain, or carry, a long-term circuit overload to a fusing termination, for example a current of 200% fuse rated capacity for a sufficient length of time to cause the fuse to open, the tin wire is incapable of sufficiently dissipating the resistance heat generated by the current flowing therethrough. As a result, the fusing link opening will occur as a localized explosion at the transverse location in the tin wire where maximum heating, in relation to localized wire heat dissipation, occurs. The explosive opening is a natural result of the resistance heat generating and heat dissipation characteristics of tin which the fuse designer must accommodate in the fuse design. This "explosive" opening is a metalized vapor created out of the vaporized tin fusing wire, which splatters out against the walls of the auxiliary tube. This metalized vapor is conductive, and electric current will continue to pass through the fuse and arc through the metalized vapor for a period of time. The arc will continue to generate until the severed ends of the fusing wires are separated a sufficient distance to create a sufficient gap therebetween which is greater than the gap-bridging power capacity of the arc. It should be appreciated that where the arc is sustained on fuse wire metal deposited on the sides of the auxiliary tube, the separation distance necessary to stop the arcing is greater than were air only present in the gap, because the electric resistance of air is several orders of magnitude greater than that of tin. If the arcing condition persists, the arc will begin burning the inside of the auxiliary tube, leading to a possible fire. In addition to the fusing deficiencies of tin, tin wires have a low tensile strength, and therefore a secondary wire made from a stronger material, such as ni-chrome, must often be used in parallel with the tin wire to support the tensile forces needed in a fuse. The use of the two parallel wires causes undesirable discontinuities in the fusing characteristics of the fuse, as both must sever to open the fuse.
In addition to tin, silver is another common fusing link material. Silver has a higher strength, but lower resistance and higher cost than tin. To compensate for the lower resistance of silver as compared to copper, the fusing link must be thicker and longer. The longer link will sometimes sag during overload conditions, causing it to contact the side of the auxiliary tube and scorch or burn the tube, creating altered fusing characteristics and the possibility of fire.